This invention relates to flexible sheet materials used for forming packages or the like for containing electrostatically sensitive components and protecting them from potentially damaging electrostatic charges, and also relates to packages formed therefrom.
As electronic components and units have become increasingly more complex, while achieving ever smaller dimensions, the problem of damage to such components due to electrostatic discharge has become of major concern. The buildup of electrostatic charges on the packages for such components can result in the discharge of a spark which can arc over or otherwise damage or destroy the components.
Similarly, the medical and pharmaceutical industries have requirements for ultraclean instruments and drug substances. Packaging of such instruments and drugs is a problem because buildup of electrostatic charges on the packaging materials attracts dust and other contaminants from the air and surroundings. Such contaminants may become mixed with the chemicals or drugs causing purity problems. Likewise, when the package is opened or the product poured out a static charge is generated on the product which may attract contaminates.
Accordingly, a number of packages and packaging materials have been developed which have antistatic properties. These antistatic packages have found wide use in a number of industries including the aerospace, chemical and pharmaceutical, and computer and electronics industries. Such packages are designed to prevent the buildup of electrostatic charges on the product contained therein, and may also be designed to provide protection against external electric fields.
For example, many packages have been fabricated of plastic films or sheets compounded with or coated with antistatic additives such as quaternary amines, amine salts or soaps, polyethylene glycols or ethers, and the like. These agents act as humectants, absorbing moisture from the air, to reduce friction within the package and static charge buildup on the package. However, such antistatic agents may not be permanent (i.e., may migrate to the surface and be lost from the plastic) and are humidity dependent.
Other packages have included one or more layers of conductive material to form a protective envelope or Faraday cage-type structure about the product. A Faraday cage may be defined as an electrostatic shield composed of a continuous mesh or series of interconnected electrical conductors which surrounds a defined volume of space. Examples of packages utilizing conducting layers include Yenni Jr., et al, U.S. Pat. Nos. 4,154,344 and 4,156,751; Petcavich, U.S. Pat. No. 4,424,900; and Dedow, U.S. Pat. Nos. 4,471,872 and 4,496,406.
However, some of these packages have no provision for closing the package, which leaves the Faraday cage protection incomplete. If adhesive tape or the like is used to close such packages, the act of stripping off the tape to reopen the package may itself create a static charge which is potentially damaging to the contents of the package. Moreover, normal flexing and handling of some prior art packages having conductive layers causes portions of the conductive layers to flake off. These flakes of material themselves are potential sources of contamination to the contents of the packages.
Accordingly, the need still exists in the art for a flexible packaging material and package which has permanent antistatic properties and which protects the contents thereof both from electrostatic charge buildup within the package as well as from external electrical fields and which has a closure means to complete the Faraday cage.